1. Field of the Invention
This invention relates to a method for the wet oxidation of a waste water containing a chemical oxygen-demanding substance (hereinafter referred to as "COD" component) in the presence of a catalyst. More particularly, it relates to a method for efficient detoxification of a waste water containing harmful oxidizable organic or inorganic substances, i.e. a COD component, by the catalytic wet oxidation of the waste water in the presence of molecular oxygen thereby effecting conversion of such organic substances into harmless substances such as carbon dioxide, water, and nitrogen.
2. Description of the Prior Art
In the methods for treatment of waste water, the biochemical method called as an "activated sludge method" and the wet oxidation method called as "Zimmermann method" have been popular.
As universally known, the activated sludge method spends a long time in the decomposition of organic substances and requires the waste water to be diluted to a concentration fit for the growth of algae and bacteria and, therefore, has the disadvantage that the facilities for the activated sludge treatment demand a large floor area for their installation. Further in recent years, particularly in urban distriots, the disposal of excess sludge arising from the treatment calls for a huge expense. The Zimmermann method effects oxidative decomposition of organic substances contained in a high concentration in an aqueous solution by introducing air under a pressure in the range of 20 to 200 atmospheres at a temperature in the range of 200.degree. to 370.degree. C. into the aqueous solution. This method requires to use a large reaction vessel because the reaction proceeds at a low velocity and consumes a long time in the decomposition of organic substances and also requires this reaction vessel to be made of a material of high durability. The method, therefore, has an economic problem in terms of the cost of equipment and the cost of operation of this equipment. Methods which use various oxidizing catalysts for the purpose of heightening the reaction velocity in the treatment have been proposed.
In the catalysts heretofore proposed for use in the method of catalytic wet oxidation, compounds of such noble metals as palladium and platinum (JP-A-49-44,556(1974)) and compounds of such heavy metals as cobalt and iron (JP-A-49-94,157(1974) have been popular. These catalysts have the aforementioned compounds deposited on spherical or cylindrical pieces of a carrier made of alumina, silica-alumina, silica gel, or activated carbon, for example. In the catalytic wet oxidation of a waste water, the waste water more often than not has the pH value thereof adjusted to a level exceeding 9 in advance of the treatment. Our study on these catalysts has revealed that, in the course of a protracted use, the catalysts suffer from loss of strength and reduction of size and, moreover, the carrier possibly succumbs to dissolution under the impact of the treatment. In the case of an alumina type catalyst, for example, the loss of strength tends to occur on account of exudation of alumina.
Recently, a method using titania or zirconia as a carrier for the purpose of solving the problem of this nature has been proposed (JP-A-58-64,188(1983)). This invention discloses a catalyst having a compound of such a noble metal as palladium or platinum and a compound of such a heavy metal as iron or cobalt deposited on spherical or cylindrical pieces of carrier made of titania or zirconia. The catalyst according with this invention is indeed recognized to possess highly satisfactory strength as compared with the catalyst using the conventional carrier. The catalysts of this class invariably come in a particulate form. They are not fully satisfactory in catalytic activity or durability. The titania type catalysts, for example, are liable to undergo loss of strength due to crystal transformation.
In some cases, the oxide of an element of the lanthanide series has been used as a catalyst. This catalyst, however, is not fully satisfactory in physical durability or moldability.
Methods using the combinations of a cerium compound with such composite oxides as TiO.sub.2 -ZrO.sub.2, TiO.sub.2 -SiO.sub.2 -ZrO.sub.2, and TiO.sub.2 -ZnO for the purpose of further improving the conventional catalysts have been proposed (JP-A-63-158,189(1988) and 1-218,684(1989) to 1-218,686(1989)). These catalysts are notably improved in durability and activity. They, however, prove at times to be inferior in initial activity to catalysts having catalytically active substances deposited exclusively on ceria.
Incidentally, in the wet oxidation of a waste water, the water is required to be treated in a very large amount. More often than not, therefore, the method of using a fixed bed of catalyst in a flow system is adopted for the reaction. Further, the waste water frequently contains solid substances. In these cases, when the catalyst is in a particulate form, the fixed bed suffers from heavy pressure loss due to the passage of the waste water therethrough and consequently fails to give to the waste water the treatment at a high linear velocity and, therefore, requires to possess a large cross section for the flow of the waste water and give a proportionate addition to the floor area for the reaction vessel. In the case of the treatment of a waste water entrailing solid substances, the resistance offered by the fixed bed to the flowing waste water in consequence of the clogging of the bed with the solid substances is increased so much as to raise the running cost of the apparatus and render lasting operation of the apparatus impracticable. Specifically, in the treatment of waste water by the catalytic wet oxidation technique, since the reaction is carried out at an elevated temperature under a high pressure, the increase in the floor area to be occupied by the reaction vessel entails a fatal problem of boosting the cost of equipment.
The fluidized bed method which fluidizes a powdery catalyst for the purpose of lowering the pressure loss caused by a catalyst bed has been proposed. This method has not yet found utility in any practical application, however, because it entails the disadvantage that the treatment requires use of a huge reaction vessel owing to the inevitable decrease in the concentration of the catalyst and the separation of the catalyst from the treated waste water is difficult.
There is also a method which effects oxidative decomposition of organic substances in a waste water at normal room temperature under normal pressure by the use of ozone or hydrogen peroxide as an oxidizing agent. JP-A-58-55,088(1983), for example, discloses a method which effects oxidative decomposition of organic substances such as humic acid contained in a waste water at 20.degree. C. under normal pressure in the absence of a catalyst by the use of ozone and hydrogen peroxide. JP-A-58-37,039(1983) discloses a method which comprises adding a surfactant to a waste water containing an organic compound possessing an aromatic ring, mixing the resultant mixture with at least one member selected from among compounds of transition metals and alkaline earth metal compounds, and then bringing ozone into contact with the produced mixture at normal room temperature under a normal pressure and therefore effecting oxidative decomposition of the organic compound. Since the former method carries out the treatment in the absence of a catalyst, it is incapable of effectively treating a waste water having sparingly oxidizable substances suspended therein. Since the latter method uses the metallic ions of a transition metal or an alkaline earth metal as a catalyst, the treated waste water cannot be released in its unmodified form from the equipment and must be purged of the metallic ions in advance of the release and, therefore, has the disadvantage that the treatment requires an extra step for aftertreatment. Both these two methods have the disadvantage that since they treat the waste water at normal room temperature under normal pressure, the treatment consumes expensive ozone in a large amount, the reaction proceeds at a low velocity, the decomposition of organic substances occurs in a low ratio, and the ozone partially escapes the reaction and the unaltered ozone necessitates a treatment for detoxification.
An object of this invention, therefore, is to provide a method for effecting efficient and lasting treatment of waste water.
Another object of this invention is to provide a method for treating waste water efficiently at a high linear velocity.
Yet another object of this invention is to provide a method for treating a waste water containing a solid substance stably at a high linear velocity for a long time.